Rotary torque detecting devices conventionally known are a mechanical type in which a coil spring is directly connected to a motor and a strain gauge type adapted to detect a pressure exerted on a body by utilizing constantan or an alloy of manganin and nickel.
In the mechanical type torque detecting device with a coil spring directly connected to a motor, torque detected is indicated by mechanical indicating means or other mechanism, and accordingly a low rate of amplification, a low detecting accuracy, and difficult connection of the device to a recorder or other equipment have been inevitable.
In the meantime, the strain gauge type torque detecting device adopts a method of detecting a change in resistance by the use of a bridge or the like; but has a shortcoming in that the output voltage is as little as 0.5 mV to 2 mV. Therefore, the design and manufacture of an amplifier becomes complicated in nature.